Steam turbine and method of measuring the vibration of a moving blade in a flow passage of a steam turbine

ABSTRACT

Steam turbine and method of measuring the vibration of a moving blade in a flow passage of a steam turbine The invention relates to a steam turbine ( 3 ) having an optical measuring system ( 60 ) for measuring a vibration of a moving blade. A transmitter ( 55 ) produces a light beam ( 61 ) which strikes the moving blade ( 27, 29, 31, 33 ) and, reflected from the latter, reaches a receiver ( 57 ). By separating the transmitter ( 55 ) from the receiver ( 57 ), a measuring angle is obtained which reduces the scattered-light effect of steam to such an extent that a reliable optical measurement of the blade vibration surprisingly becomes possible. The invention also relates to a method of measuring the vibration of a moving blade ( 33 ) in a flow passage ( 19, 21, 23, 25 ) of a steam turbine ( 3 ).

[0001] The invention relates to a steam turbine having a flow passage inwhich a moving blade is arranged, and having a measuring system formeasuring a vibration of the moving blade. The invention also relates toa method of measuring the vibration of a moving blade in a flow passageof a steam turbine.

[0002] The technical field of the invention is the measurement of bladevibrations in fluid-flow machines. Moving blades in fluid-flow machinesare subjected to high loads. A vibration may be induced in them onaccount of alternating stresses, this vibration, if it lies in thevicinity of a natural vibration in the respective blade, leading toespecially high mechanical stresses of this blade. In order to detectsuch especially high loads in good time, the vibration states of theblades are measured in different ways.

[0003] U.S. Pat. No. 4,996,880 discloses a steam turbine and a method ofmeasuring the vibration of a moving blade in the flow passage of a steamturbine. Here, the vibration of the moving blade is measured by means ofan acoustic signal. A Doppler displacement which is caused by themovement of the moving blade is measured with an acoustic sensor.Depending on the vibration state of the moving blade, a characteristicDoppler signal is obtained, so that, in inverse relationship to theDoppler signal, the vibration state of the moving blade can be deduced.

[0004] U.S. Pat. No. 4,518,917 shows a method of measuring the vibrationstate of moving blades, in which method the distance of the blades fromthe surrounding casing is measured. An impedance dependence of sensorsarranged in the casing is utilized in this case. Depending on thedistance of a moving blade from the sensor, an impedance change isobtained.

[0005] A further method of measuring the vibration of a moving blade hasbeen disclosed by U.S. Pat. No. 4,934,192. Here, an axial deflection onaccount of an axial vibration of the moving blade is measured by twosensors being arranged symmetrically over a prominence on the tip of themoving blade in the rest state of the latter. The sensors are eachdesigned as an electrical winding in which a voltage is induceddepending on the distance from the prominence on the tip of the movingblade. In the event of an asymmetrical arrangement of the sensorsrelative to the tip of the moving blade, this asymmetrical arrangementoccurring on account of an axial vibration of the moving blade, adifferential signal is produced which characterizes the blade vibration.

[0006] In the paper “A Review of Analysis Techniques for Blade TipTiming Measurements”, S. Heath, M. Imregum, ASME publication 97/GT/218,presented at the International Gas Turbine and Aeroengine Congress andExhibition, Orlando, Fla., USA, May 2, 1997, a method of measuring theblade vibration by means of a laser is described. However, this methodrelates solely to gas turbines.

[0007] Said methods for the non-contact measurement of the vibration ofa moving blade are either comparatively inaccurate or require amagnetizable material for the moving blade or the awkward and unreliableplacement of a magnetic marking on the moving blade. Accordingly, anobject of the invention is to specify a steam turbine in which themeasurement of the vibration of a moving blade is possible in anon-contact and reliable manner largely independent of properties of themoving blade. A further object of the invention is to specify acorresponding method of measuring the vibration of a moving blade.

[0008] According to the invention, the object which relates to a steamturbine is achieved by a steam turbine having a flow passage in which amoving blade is arranged, and having a measuring system for measuring avibration of a moving blade, the measuring system comprising atransmitter for emitting a light beam to the moving blade and a receiverfor receiving the light beam reflected from the moving blade, and thetransmitter being separate from the receiver.

[0009] The use of an optical measuring system for measuring thevibration state of a moving blade in a flow passage of a steam turbinehas hitherto not even been taken into consideration. This is due to thefact that, according to the prevailing opinion, the steam flowing in theflow passage makes optical detection of the moving blade virtuallyimpossible on account of high scatter in the steam. In this case,however, measuring systems have hitherto always been based on atransmitter and receiver combined in a unit, so that in principlemeasurements are taken in backscatter. Such systems are used, forexample, for measuring blade vibrations of gas turbines. They offer theadvantage that only the casing part defining the flow passage need beinterfered with for fitting the transmitter and receiver. According tothe findings of the invention, such an optical measuring system may nowalso be used in a steam turbine for measuring a blade vibration if therigid concept of the coupled transmitter/receiver unit is dispensedwith. This is because, in a suitable arrangement of transmitter andreceiver, the proportion of scattered radiation which is caused by thesteam can now be kept so low that a reliable measurement of bladevibration is made possible.

[0010] The transmitter and receiver need not be designed as a completelylight-producing or light-converting unit; they may also be designed, forexample, as a glass fiber cable and direct a light beam from a lightsource or to a converting unit, such as a photocell for instance.

[0011] The transmitter is preferably designed for emitting a laser beam.On account of its monochromasy and low divergence, a laser beam isespecially suitable for the measurement.

[0012] The transmitter and receiver are preferably arranged in such away that the transmitted light beam and the reflected light beam enclosean angle of reflection of at least 45° with one another. The angle ofreflection is also preferably greater than 90°. In such a large-angledarrangement, a good ratio of light beam reflected directly into thereceiver to scattered radiation caused in the steam is obtained, sincethe proportion of scattered radiation drops with a larger angle.

[0013] The transmitter is preferably set in such a way that thetransmitted light beam illuminates an area of less than 1 mm² on themoving blade. High focusing or a low divergence of the light beamlikewise encourages a low proportion of scattered radiation.

[0014] The receiver, in a casing defining the flow passage, ispreferably arranged so as to be set back from the flow passage in such away that, apart from the directly reflected light beam, at most a smallproportion of scattered radiation reaches the receiver. By the receiverbeing set back in the casing, a diaphragm, as it were, is constructed,and this diaphragm essentially allows only such light to reach thereceiver which spreads in a virtually rectilinear direction from theilluminated area on the moving blade to the receiver. As a result,scattered radiation spreading at other angles is mostly screened offbefore entering the receiver.

[0015] The moving blade is preferably made of a nonmagnetic material.The moving blade is also preferably made of a titanium-based alloy.Here, the expression “non-magnetic” means that the material of themoving blade has no appreciable ferromagnetic properties. Especially inthe case of such a material, a simple non-contact measurement by meansof magnetic induction is ruled out. Such a material is, for example, atitanium-based alloy, which are used in new generations of steamturbines, in particular when the moving blades of the last stages oflow-pressure parts are very large. However, such large blades especiallyare susceptible to vibration excitation and are loaded to a considerableextent by vibrations. Here, especially, a reliable monitoring system formeasuring the blade vibration states must therefore be used. By means ofthe optical monitoring of the blade vibrations, this is also possiblefor such blades in a simple and reliable manner.

[0016] According to the invention, the object which relates to a methodis achieved by a method of measuring the vibration of a moving blade ina flow passage of a steam turbine, a light beam being directed onto themoving blade and being reflected from the latter at an angle ofreflection greater than 45° and directed to a receiver, and the bladevibration being calculated from the signal thus received.

[0017] The advantages of such a method follow in accordance with theabove statements in relation to the advantages of the steam turbine.

[0018] The angle of reflection is preferably greater than 90°.

[0019] The light beam used is preferably a laser beam.

[0020] The light beam is preferably directed onto a reflecting surface,lying on the moving blade, in such a way that the illuminated part ofthe reflecting surface is less than 50 mm².

[0021] The moving blade is preferably made of a non-magnetic material,also preferably of a titanium-based alloy.

[0022] The invention is explained in more detail by way of example withreference to the drawing. In the drawing, partly schematically and notto scale:

[0023]FIG. 1 shows a steam turbine, and

[0024]FIG. 2 shows a measuring system for measuring the vibration of amoving blade in a steam turbine.

[0025] The same reference numerals have the same meaning in thedifferent figures.

[0026]FIG. 1 shows a steam turbine 3. A high-pressure part 9, anintermediate-pressure part 11 and a double-flow low-pressure part 13 arearranged one behind the other on a common shaft 5 between bearings 7.The double-flow low-pressure part 13 consists of a first low-pressurehalf 15 and a second low-pressure half 17. The low-pressure part 13 hasa low-pressure casing 14. The intermediate-pressure part 11 has anintermediate-pressure casing 12. The high-pressure part 9 has ahigh-pressure casing 10. The high-pressure part 9 has a high-pressureflow passage 19. The intermediate-pressure part 11 has anintermediate-pressure flow passage 21. The low-pressure part 13 has afirst low-pressure flow passage 23 in the first low-pressure half 15 anda second low-pressure flow passage 25 in the second low-pressure half17. Moving blades 27 are arranged in successive moving-blade rings inthe high-pressure flow passage 19. Intermediate-pressure moving blades29 are arranged in successive moving-blade rings in theintermediate-pressure flow passage 21. Moving blades 31 are arranged insuccessive moving-blade rings in the low-pressure flow passage 23 of thefirst low-pressure half 15. Moving blades 33 are arranged in successivemoving-blade rings in the low-pressure flow passage 25 of the secondlow-pressure half 17 of the low-pressure part 13.

[0027] During operation of the steam turbine 3, live steam is fed to thehigh-pressure part 9 via a high-pressure steam feed line 41, this livesteam partly expanding in the high-pressure part 9 and converting energyinto energy of rotation of the shaft 5 in the process. The partlyexpanded steam is then directed via an intermediate-pressure feed line43 to the intermediate-pressure part 11, where it expands further andtransmits further energy of rotation to the shaft 5. The steam is thendirected in double flow via a low-pressure steam feed line 45 to thelow-pressure part 13 in such a way that it flows in parallel through thefirst low-pressure half 15 on the one hand and through the secondlow-pressure half 17 on the other hand. In the process, the steamexpands further and further energy of rotation is transmitted to theshaft 5. The largely expanded steam is then fed via discharge lines 47to a condenser (not shown). Also not shown for the sake of clarity areguide blades, which are arranged alternately to the moving blades in theaxial direction in the flow passages.

[0028] The steam flowing past the moving blades may lead to vibrationsbeing induced in the moving blades. This applies in particular to thevery large moving blades 31, 33 of the low-pressure part 13, to beprecise in particular to the last stages of the low-pressure part 13.Such vibrations may result in the moving blades 31, 33 being loaded insuch a way that the service life is reduced. In order to detect this ingood time, the vibration state of each moving blade 31, 33 is monitored.With previous measuring techniques, this was only possible in acomplicated manner, or virtually not all in the case of non-magneticmoving blades 31, 33.

[0029]FIG. 2 shows a detail of the second low-pressure half 17 ofFIG. 1. A measuring system 60 is arranged in the low-pressure casing 14.The measuring system 60 comprises a transmitter 55 and a receiver 57. Alight beam 61, formed by a laser beam, is directed by the transmitter 55into the flow passage 25 and onto a moving blade 33. The moving blade 33has a shroud band 51 which has a beveled reflecting surface 53 towardthe transmitter 55. From the reflecting surface 53, the light beam 61 isdirected as reflected light beam 63 to the receiver 57. The transmitter55 and the receiver 57 are separated from one another in such a way thatan angle α greater than 90° is obtained between the incident light beam61 and the reflected light beam 63. On the reflecting surface 53, thelight beam 61 illuminates a part area 65 which is less than 1 mm².

[0030] Due to the separate arrangement of transmitter 55 and receiver57, it is possible to take a reflection measurement even in the flowpassage 25, through which steam flows, of the steam turbine 3. Whereasaccording to the prevailing opinion the steam should result in far toomuch scattered light being produced, as a result of which an opticalmeasurement would not be realizable, the arrangement shown, on accountof the comparatively large angle α, offers the possibility of a reliableoptical measurement without interference by scattered light. This isfurther assisted by a low divergence of the light beam 61 and by thereceiver 57 being set back in the low-pressure casing 14, as a result ofwhich a channel-like diaphragm is produced which largely screens offscattered light.

1. A steam turbine (3) having a flow passage (25) in which a movingblade (33) is arranged, and having a measuring system (60) for measuringa vibration of the moving blade (33), characterized in that themeasuring system (60) comprises a transmitter (55) for emitting a lightbeam (61) to the moving blade (33) and a receiver (57) for receiving thelight beam (63) reflected from the moving blade (33), the transmitter(55) being separate from the receiver (57), and the transmitter (55) andreceiver (57) being arranged in at least one casing part (10, 12, 14,15, 17) of the steam turbine (3).
 2. The steam turbine as claimed inclaim 1, in which the transmitter (55) is designed for emitting a laserbeam (61).
 3. The steam turbine (3) as claimed in claim 1, in which thetransmitter (55) and receiver (57) are arranged in such a way that thetransmitted light beam (61) and the reflected light beam (63) enclose anangle of reflection (α) of at least 45° with one another.
 4. The steamturbine (3) as claimed in claim 3, in which the angle of reflection (α)is at least 90°.
 5. The steam turbine (3) as claimed in claim 1, inwhich the transmitter (55) is set in such a way that the transmittedlight beam (61) illuminates an area (65) of less than 1 mm² on themoving blade (33).
 6. The steam turbine as claimed in claim 1, in whichthe receiver (57), in a casing (14) defining the flow passage (25), isarranged so as to be set back from the flow passage (25) in such a waythat, apart from the directly reflected light beam (63), at most a smallproportion of scattered radiation reaches the receiver (57).
 7. Thesteam turbine (3) as claimed in claim 1, in which the moving blade (33)is made of a non-magnetic material.
 8. The steam turbine (3) as claimedin claim 7, in which the moving blade (33) is made of a titanium-basedalloy.
 9. A method of measuring the vibration of a moving blade (33) ina flow passage (25) of a steam turbine (3), characterized in that alight beam (61) is directed onto a moving blade (33) and is reflectedfrom the latter at an angle of reflection (α) greater than 45° anddirected to a receiver (57), the blade vibration being calculated fromthe signal thus received.
 10. The method as claimed in claim 9, in whichthe angle of reflection (α) is greater than 90°.
 11. The method asclaimed in claim 9, in which the light beam (61) used is a laser beam.12. The method as claimed in claim 9, in which the light beam (61) isdirected onto a reflecting surface (53), lying on the moving blade (33),in such a way that the illuminated part (65) of the reflecting surface(53) is less than 50 mm².
 12. The method as claimed in claim 9, in whichthe moving blade (33) is made of a non-magnetic material.
 13. The methodas claimed in claim 12, in which the moving blade (33) is made of atitanium-based alloy.